1. Field of the Invention
The present invention relates to a novel and inventive axle driving apparatus for running vehicles, comprising a combination of two hydraulic transmissions (hereinafter referred to as HST), one for forward-rearward drive switching and changing speed, and the other for steering, wherein a steering device such as a steering wheel is manipulated in the same direction to turn the vehicle in the right or left direction regardless of whether the vehicle is running forward or rearward.
2. Related Art
A running vehicle having a configuration in which a pair of right and left driving axles are driven separately by a pair of HSTs is well known. One example thereof is disclosed in U.S. Pat. No. 4,782,650. In such a vehicle, the output rotational speeds of the two HSTs are rendered to coincide with each other when the vehicle is running straight, and are differentiated when the vehicle turns right or left.
In the conventional vehicle having two HSTs as described above, the HSTs, together with the link mechanism and the axles associated with the HSTs, require high precision in fabrication and assembly to accomplish an exact coincidence of the output rotational speed of the HSTs when the vehicle is driving straight. Also, considerable time is required for output adjustment of the left and right driving axles before shipment. Assuming that the pump or motor capacity is different between the two HSTs due to a fabrication error or the like, it is possible to make adjustments to assure the same rotational speed of the right and left axles when the vehicle is running straight. When the vehicle turns right or left, however, the steering feel is different between right and left turns, thereby making the vehicle difficult to maneuver.
With such a vehicle, a pair of HSTs are normally juxtaposed on right and left sides, which in turn increases the vehicle width, resulting in difficulty for the vehicle to make a small turn. Also, in the case of a specialty vehicle, the job efficiency is adversely affected.
One solution to the problem of an axle driving apparatus having two combined HSTs is a configuration in which one HST is used for speed change with a pair of axles power-coupled to a normal differential arrangement for differential coupling and the other HST is used for steering with an output rotation thereof applied through another steering differential arrangement to the two axles for rotation in opposite directions, so that at the time of steering, one axle is increased in speed by the output from the steering differential arrangement, and the other axle is decreased in speed. This turns the vehicle toward the side of the decelerating axle.
In this configuration, the rotational direction of the steering HST is switched according to the direction in which the steering wheel or other steering means is manipulated. Even when the steering wheel is manipulated in the same direction, the axle operatively coupled to the speed-change HST is rotated in the opposite direction depending on whether the vehicle is running forward or rearward. Thus, the axle increased in speed when the vehicle is running forward is decreased in speed when the vehicle is running rearward, and vice versa. In a configuration in which the steering means is a steering wheel, counterclockwise rotation of the steering wheel in the forward drive mode decelerates the left axle and accelerates the right axle to turn the vehicle to the left. If, however, the steering wheel is rotated counterclockwise when the vehicle is running in reverse, the left axle is accelerated while the right axle is decelerated, thereby causing the vehicle to turn to the right. In other words, for the vehicle to turn left while running in reverse, the steering wheel is required to be rotated clockwise. This gives the driver a driving feel that is considerably different from one normally experienced in passenger cars. This may lead to an increase in traffic accidents due to a driving error.